Coupling radiation between an optical fiber and a photonic integrated circuit, such as a planar photonic circuit using silicon technology, can be challenging for several reasons. Three important challenges in light-chip coupling are achieving a good coupling efficiency between an optical fiber and the integrated circuit, achieving a high bandwidth, and dealing with implementation complexity.
Grating-based fiber-chip couplers may provide a versatile solution which is relatively simple to implement and allows wafer-scale optical testing. Various advantages of grating-based fiber-chip couplers include the freedom to place the coupler anywhere on the chip, no facet polishing requirement, and the ease of implementation of optional functionality such as power or polarization splitting.
In a conventional grating-based fiber-chip coupler as known in the art, a good light coupling in and out of a planar waveguide may be achieved by means of a grating for implementing a cyclic local effective refractive index modulation. For example, such grating may comprise a periodic line and space structure forming alternating high effective refractive index and low effective refractive index regions. The grating structure in couplers known in the art may for example be implemented by metallic lines, shallow-etched grooves or slanted grooves.
However, despite the practical advantages of state-of-the-art grating couplers, the operational bandwidth offered by such couplers may be relatively limited, and the coupler may be sensitive to input polarization. Furthermore, difficulties in achieving good fabrication tolerances in such couplers may also make them less attractive for industrial application.
Particular fiber-to-chip grating couplers known in the art comprise a sub-wavelength grating. For example, Chen and Tsang disclosed in “Nanoholes Grating Couplers for Coupling Between Silicon-on-Insulator Waveguides and Optical Fibers,” published in IEEE Photonics Journal 1(3), a high-efficiency grating coupler for coupling between the TE mode of a nanophotonic wire waveguide and a single-mode optical fiber. This coupler uses an array of nanoholes instead of a conventional grating, and may thus be advantageously manufactured using the same photolithography mask and etching process as is used for the silicon-on-insulator waveguide. The sub-wavelength structures may reduce the effective refractive index step without substantially distorting the phase front of the propagating light wave, thus reducing losses by, for example, reflection off the grating. However, the coupling efficiency of such coupler may be limited by the directional nature of diffraction, which furthermore can be improved to some extent by adding a substrate mirror.